1. Field of the Invention
The present invention relates generally to cathode ray tube (CRT) devices, and more particularly to a circuit which reduces extremely low frequency (ELF) electric fields emanating from CRT devices through production of a gain-modified impedance at the CRT anode.
2. Description of the Background Art
Changes in information displayed on a CRT device or user adjustment of CRT display controls, such as brightness and contrast, unpredictably alter the CRT electron beam current, giving rise to corresponding variations in the CRT anode voltage. As a result, the anode acts as a radiating element, producing extremely low frequency (ELF) electric fields. The presence of ELF electric fields may pose a potential health risk to CRT users, although no definitive conclusion in this regard has been made to date. Since a user may be required to view the CRT screen for several hours each day, potential health effects must be minimized.
In light of this, manufacturers of CRT-based display devices have taken various approaches toward the reduction of ELF electric fields. In a first approach, shielding is selectively used on the CRT in order to attenuate the electric field. One common type of shielding consists of a metal layer applied to a casing which encloses the CRT. On the front of the CRT display, shielding requires use of a specialized transparent coating capable of attenuating electric fields. Such coatings typically degrade image quality or significantly increase the cost of CRT manufacture.
Another approach utilizes a circuit of the type shown in FIG. 1, in which the unwanted ELF electric field is capacitively sensed, inverted, and amplified in order to produce an inverted replica of the voltage waveform at the anode. This signal is then radiated back towards the CRT through a radiating strip or plate. In order for cancellation to be effective, the location of the ELF electric field sensing means and the gain provided by the amplification means must be properly chosen in order to account for possible asymmetries in the magnitude of the ELF electric field surrounding the CRT. A similar consideration applies to the cancellation signal radiating means. In addition, nonlinearity and phase distortion present in the amplification means must be minimized. These considerations complicate CRT design and manufacture.
Since the CRT anode radiates ELF electric fields as a result of anode voltage variation, other approaches for reducing the magnitude of ELF electric fields rely upon reducing this voltage variation. This type of approach offers the added benefits of reduced geometric image distortion and greater image stability. One such method involves the use of a voltage-regulated power supply, in which electronic circuitry coupled to the power supply ensures relatively constant voltage output at the CRT anode. A circuit which functions in this manner is described in U.S. Pat. No. 5,043,598; this circuit regulates a CRT anode voltage by subtracting a variable voltage from the voltage present across a flyback transformer. A major disadvantage of this circuit is that the dynamic range of the regulator must accommodate unit-to-unit output voltage variation resulting from the flyback transformer circuitry, in addition to the voltage variation produced by changes in the anode current. This circuit additionally suffers from significant power dissipation, as do most regulators. Circuitry required for regulation of CRT anode voltages also significantly adds to the cost of a CRT anode power supply. Due to these disadvantages, the majority of CRT-based display devices utilize unregulated anode power supplies.
An additional approach which addresses reduction of CRT anode voltage variation as a means for reducing ELF electric fields, involves minimization of the impedance connected to the CRT anode. In the circuit of FIG. 2, this is accomplished through the use of a passive bypass capacitor possessing very high capacitance. Unfortunately, such a capacitor is much more expensive and physically much larger than one having lower capacitance, again adversely affecting the economic aspects of CRT design and manufacture. Use of this type of capacitor also increases the potential for damage to the CRT due to a sudden electrical discharge, also known as arcing.
What is needed is an efficient and cost effective circuit for reducing ELF electric fields radiated from the anode of a CRT, while also reducing geometric image distortion in the CRT display.